Acrylonitrile-butadiene-styrene (ABS) copolymer resins have been widely used for various products such as inner parts of automobiles, office automation (OA) devices, construction materials and the like, in addition to small household and personal items/amenities.
Recently, demand for a non-glossy resin which is used for inner parts of automobiles, housings for electronic appliances and the like is increasing. The use of non-glossy resins without a coating to protect the environment is becoming a widespread trend.
Non-glossy ABS resin can be generally prepared using one of three methods. The most widely used method uses a non-gloss additive-matting agent such as an inorganic filler, an acrylic resin or a cross-linked styrene resin. Another method forms a rough surface by controlling the rubber particle size of a dispersed phase in an ABS resin. The rough surface can scatter light, which results in low gloss. Another method removes gloss during a post processing procedure. In addition, there is a method of obtaining non-gloss effect by using an etching mold during an injection molding process or a coating process.
Although using a non-gloss additive-matting agent can be a convenient method for obtaining non-glossy characteristics, there is a drawback in that homogeneous gloss may not be obtained depending on the distribution of the additive. In addition, using an etching mold during an injection molding process or a coating process can increase production costs and is not environmentally friendly. On the other hand, forming a rough surface by controlling rubber particle size has an advantage in that the method can provide high quality non-glossy characteristics without an additional process in a continuous bulk polymerization.
In order to obtain low gloss by controlling rubber particle diameter, various conditions in the polymerization process must be modulated or controlled appropriately. Bulk polymerization or solution polymerization is more suitable than emulsion polymerization for controlling rubber particle diameter. It is impossible to obtain a rubber particle diameter of 1 μm or more in the ABS resin prepared using emulsion polymerization. In contrast, high-impact polystyrene (HIPS) prepared using bulk polymerization can easily have a rubber particle diameter of about 2 to about 3 μm. An ABS resin prepared using bulk polymerization or solution polymerization usually has a particle diameter of 1 μm or less.
Three factors play an important role in controlling a rubber particle diameter when preparing an ABS resin using a continuous process such as bulk polymerization or solution polymerization (see Rubber-Toughened Plastics, C. Keith Riew, Advances in Chemistry series 222, 1987). Three factors for controlling rubber particle size are shear force generated by stirring force of a reactor, viscosity ratio between a dispersed phase and a continuous phase, and interfacial force between the two phases.
In general, as the stirring rate of a reactor increases, the shear force becomes stronger and thus rubber particle size becomes smaller. However, if the stirring rate is too fast, dispersed particles may be reunited and the rubber particle size may become larger. With regard to the viscosity ratio between a dispersed phase (rubber) and a continuous phase (SAN), when the viscosity ratio, namely, (viscosity of a dispersed phase)/(viscosity of a continuous phase) increases, the rubber particle size increases. Further, when the interfacial force between a dispersed phase and a continuous phase increases, particle size decreases.
European Patent Publication No. 412,801 discloses an ABS resin having a small particle diameter of about 0.5 μm or less and a large particle diameter of about 1 to 2 μm. European Patent Publication No. 277,687 discloses an example in which a branch type rubber is used to decrease solution viscosity.
Japanese Patent Publication No. 8-120032 discloses an ABS resin having a rubber particle size similar to that of EP 412,801, which exhibits improved surface gloss by going through the extrusion process.
U.S. Pat. No. 5,250,611 discloses a method for preparing an ABS product having good physical properties by using a low temperature free radical initiator and a tube reactor, and U.S. Pat. No. 5,223,577 discloses a method for continuously preparing an ABS resin by using SBR rubber. However, these methods are used only for preparing products with an average particle diameter of 1 μm or less.
U.S. Pat. No. 4,277,574 discloses an ABS product having a bimodal rubber particle size of 0.01 to 0.5 μm (small) and 0.7 to 10 μm (large). The invention, however, is not directed to the non-glossy ABS resin, since the size of the large rubber particle used in the invention is substantially limited to 0.8 to 3 μm stated to provide a good balance of physical properties.
As such, conventional methods focus on making a rubber particle size of less than 1 μm or partly introducing a rubber having a large particle diameter of 1 to 3 μm to improve physical properties. Although the products prepared from these methods have advantages such as good color, purity, low cost, and so forth, compared to ABS products prepared by a conventional emulsion polymerization, they are not desirable in terms of physical properties.